Electronic circuits can be damaged by electrostatic potentials, or just high voltage transients in general. This is particularly true for field effect transistors.
Protection can, for example, be provided by placing reverse biased diodes between a node which is to be protected and power supply rails. As used herein, “node to be protected” will refer to any one or more elements, conductors or terminals that are to be protected against electrostatic damage.
A disadvantage with diodes is that they start conducting as soon as the necessary forward bias voltage, typically 0.7 volts, has been achieved. Thus if it is desired to protect against overvoltage events of positive and negative going polarity, then it is difficult to prevent the diodes from conducting when legitimately supplied voltages might fall outside the supply rail voltage of the device incorporating or attached to the node to be protected. Thus, it is desirable to set a “protection voltage” at which protection is initiated independently of the supply voltage.
A further disadvantage with diodes is that they exhibit a resistive current-voltage characteristic once they switch on. This means that the time taken to conduct an electrostatic discharge safely away might be unnecessarily long. Also since the rate of current flow is restricted by the resistance of the diode, the peak voltage experienced by the protected node might be unnecessarily high. This may be especially significant for cases in which the overvoltage events result from the presence of inductive components as opposed to electrostatic discharges. Diodes are not the only form of overvoltage protection that can be employed, but they serve to illustrate some of the issues that need to be overcome.